The best way to go about it, is to first install the Simple Image Sizes plugin ( http://wordpress.org/extend/plugins/simple-image-sizes/ ). The plugin will give you complete control over all images in the theme -from aspect ratio, to width, whether to crop the image, and even has a built in thumbnail regenerator.

After installing the plugin, you will find your Settings > Media page in the backend to contain the various sizes. Then simply find the dimensions you want to change. Save. Regenerate the images and see if you like what you see or want to tweak the image sizes further.